How do I check the parallelism of electrode?

In the case of spot welding, etc., if a certain degree of parallelism is obtained, the welding qualities are not greatly affected. Therefore, it is acceptable to visually check the parallelism. However, parallelism is important in the case of projection welding.
To check the parallelism, check the parallelism between the top and bottom electrode by sandwiching a sheet that discolors when pressure is applied, called a pressure-sensitive sheet, etc.